The invention relates to an electric motor, particularly, an electronically commutated direct current motor.
The invention relates to the field of brushless electric motors with permanent magnets and in particular direct current (DC) motors which can be configured as either inner rotor motors or outer rotor motors. Inner rotor motors feature a rotor assembly which includes a rotor shaft and one or more permanent magnets arranged on the rotor shaft as well as a stator assembly which features, for example, a stator composed of plates having phase windings. The rotor assembly is fitted in a coaxial, concentric manner in the stator assembly. In an outer rotor motor, the rotor unit surrounds the stator concentrically.
FIG. 1 schematically illustrates the basic structure of an inner rotor motor, with a housing 4 which contains the stator assembly 8, the rotor assembly 6 as well as the bearings 16, 18 to rotatably support the rotor assembly. The stator assembly 8 includes stator plates 55 and phase windings 60 and encloses an inner space into which the rotor assembly 6 can be inserted. The bearings 16, 18 for the rotor assembly can, for example, be integrated into flanges 24 or end caps 20 on the motor housing 4.
FIG. 2 shows a schematic sectional view through an exemplary two-phase DC motor having a stator assembly and a rotor assembly enclosed concentrically by the stator assembly. The stator assembly is illustrated schematically by a stator plate 52 which features four electrical poles 53 and four stator slots 54, whereby two phase windings 62, 64 of the two-phase DC motor are equally distributed between and wound on the electrical poles 53. The electrical poles are also referred to as hammers. The number of electrical poles is determined by the number of hammers or the number of stator slots on the stator assembly. The rotor assembly is illustrated schematically by a permanent magnet 14 with a magnetic pole pair 42, 44.
In the embodiment illustrated the windings 62, 64 of the first phase and those of the second phase lie opposite each other as shown in FIG. 2. Basically, the geometric offset of the phases equals 360 electrical degrees divided by the number of phases and thus in the present embodiment equals 360 electrical degrees/2=180 electrical degrees. The number of windings 62, 64 of one phase is thus equal to the number of stator slots divided by the number of phases.
The phases of the DC motor according to the embodiment shown in FIG. 2 generate the torque shown in FIG. 3 when constantly energized.
As shown in FIG. 3, there is a rotor position in two-phase DC motors in which torque cannot be generated by either energizing phase 1 or energizing phase 2. This situation is illustrated in FIG. 3 showing the “critical point”. The torque T delivered by the two-phase DC motor fluctuates between 0 and a maximum value Tmax. This fluctuation is referred to as “torque ripple”, i.e., a ripple created during the energizing process.
FIG. 4 shows a schematic sectional view through a three-phase DC motor with a stator assembly which has six electrical poles 56 and six corresponding stator slots 57 as well as a rotor assembly arranged coaxially in the stator assembly having a permanent magnet 14 with two magnetic pole pairs 46, 48. Corresponding to the three phases of the DC motor, three windings 66, 67, 68 of the first, second and third phase, respectively, are placed in the slots 57 of the stator plate 55, staggered with respect to each other. The number of windings per phase is equal to the number of slots (6) divided by the number of windings (3) and is thus 2. FIG. 5 shows the related torque profile generated by the three-phase DC motor.
As shown in FIG. 5, the geometric offset of the three phases is:360 electrical degrees/number of phases=120 electrical degrees.
Thus, in the three-phase DC motor, there are no torque gaps and torque fluctuates between a minimum and a maximum torque value Tmax, Tmin, which, however, still causes a certain torque ripple to be created. FIG. 5 also shows that in specific rotor positions (Δt10, Δt20, Δt30), the individual phases do not generate any torque.
U.S. Pat. No. 5,973,426 describes a three-phase electric motor in which the permanent magnet is divided into three axial sections which deviate 120° relative to each other. The purpose of the arrangement described is to disclose a highly efficient small-scale motor. However, this reference does not deal with the problems of torque ripple or torque gaps.
WO 99/57795 describes a motor arrangement having two independent; operable motors on a common shaft. The purpose of this arrangement is to keep the motor operating even during failures in the excitation switch.
In order to reduce the problem of torque ripple and to eliminate cogging torque, it has been suggested in the prior art to provide a rotor with a permanent magnet whose poles are obliquely magnetized to prevent abrupt switching between the individual phases. This oblique magnetization of the rotor poles, however, generates an axial load component and thus a loss of torque.